Exiting from Screen zoom by pressing "Escape" or "Backspace" key.
[Issue] https://bugs.tizen.org/jira/browse/TT-225
[Problem] User cannot exit the screen zoom.
[Cause] Screen zoom is not in ViewManager stack, so it can't be supported
by onBackPressed callback.
[Solution] Added boost signal in PlatformInputManager on "Escape"
and "Backspace" keys.
[Verify] Launch browser > Open web page > More Menu > Screen zoom
Zoom up/down the page. Then press "escape" -> Screen zoom should
disappear and the page should return to normal scale.
Change-Id: Ica7a37a9fe15f3bb27bf4f54dce53238b90cb7b5